Equilibrium Constant Expression

\[K=\frac{[C]^c[D]^d}{[A]^a[B]^b}\]

Variables

Kequilibrium constant

Aequilibrium concentration of reactant A (mol/L)

Bequilibrium concentration of reactant B (mol/L)

Cequilibrium concentration of product C (mol/L)

Dequilibrium concentration of product D (mol/L)

astoichiometric coefficient of A

bstoichiometric coefficient of B

cstoichiometric coefficient of C

dstoichiometric coefficient of D

Description

What is this formula?

The equilibrium constant expression relates the concentrations of reactants and products at chemical equilibrium according to the balanced reaction equation.

For a general reaction:

aA + bB ⇌ cC + dD

the equilibrium constant is defined by the ratio of product activities to reactant activities raised to their stoichiometric coefficients.

When to use it

Use this formula to determine equilibrium concentrations, evaluate reaction extent, compare reaction conditions, and analyze reversible chemical systems.

Example

Reaction:

N2 + 3H2 ⇌ 2NH3

Given:

[N2] = 0.50 mol/L

[H2] = 0.40 mol/L

[NH3] = 0.80 mol/L

Formula:

K = [NH3]^2 / ([N2][H2]^3)

Substitution:

K = (0.80)^2 / (0.50×0.40^3)

Result:

K = 20.0

The equilibrium strongly favors ammonia formation.

Applications

Chemical equilibrium analysis

Reaction engineering

Industrial chemistry

Thermodynamics

Chemical process design

Note

The rigorous definition of the equilibrium constant uses activities rather than concentrations. In many practical calculations, concentrations are used as an approximation. This approximation is most accurate in dilute systems where activity coefficients are close to unity. Different forms such as Kc, Kp, and Ka are specific applications of the same equilibrium principle.